This invention generally relates to toolholders for use with cutting tools, and is specifically concerned with to a rotatable toolholder having a stationary, through-center coolant feed system for providing a non-rotating stream of coolant to a rotating metal cutting tool.
Cutting tools used for such operations as drilling, milling, or reaming metal workpieces frequently require the application of a coolant at the interface between the workpiece and the cutting surfaces of the tool. The use of a coolant advantageously reduces the heat generated by the machining operation by reducing the friction between the tool and the workpiece, and by transferring the generated heat from the cutting interface to the coolant. Such heat reduction is important as the generation of excessive heat prematurely dulls the cutting tool, thus lowering the quality of the cuts produced by the tool, and shortening its useful life span. Excessive heat can also damage the workpiece.
Systems for providing coolant at the interface between such cutting tools and their respective workpieces are known in the prior art. While some of these systems operate by directing a stream of coolant from one side of the cutting action, there is an increasing demand for systems that direct the coolant through the center of the cutting tool directly to the point of contact between the workpiece and the cutting surfaces of the tool. Such through-center coolant systems allow the cutting tool to be rotated at a faster cutting speed while simultaneously achieving a finer and cleaner cut.
Prior art toolholders having such through-center coolant systems generally comprise a toolholder shank having a proximal end that is detachably connectable to the drive shaft of a turning tool, and a distal end that is connected to a tool coupling, which may be a collet-type coupling arrangement capable of firmly grasping the shank end of a cutting tool. A stationary housing envelops the toolholder in the area between the distal end of the toolholder shank and the proximal end of the toolholder coupling. Bearings are provided at the interfaces between the housing and the toolholder so that the toolholder may rotate with respect to the stationary housing. Dynamic fluid seals are also provided at the interfaces between the housing walls, the shank and tool coupling so that the housing can contain pressurized liquid coolant while the toolholder rotates. The proximal end of the toolholder shank includes a diametral bore that communicates with an axial bore present in the tool coupling. Cutting tools used in conjunction with such prior art coolant system include a through-center, coolant conducting bore extending from their shank ends all the way to their cutting ends.
In operation, when pressurized coolant is supplied to the interior of the stationary housing, liquid coolant is forced through the diametral bore of the toolholder shank and then through the axial bore in the tool coupling where it is received by the coolant conducting bore located through the center of the tool itself. The bore in the tool conducts the coolant to the interface between the cutting end of the tool and the workpiece. To conserve the coolant (which is often a mixture of water and ethylene glycol) many prior art systems collect spent coolant and recirculate it back to the source of pressurized coolant. A filtration unit is provided in the recirculation path of the coolant to remove metal chips and particles and other debris formed as a result of the cutting operation.
While such prior art through-center coolant systems have proven to be effective in providing a steady stream of coolant to the exact point of contact between the rotating cutting tool and a metal workpiece, the applicant has noted a number of areas where such systems could be improved in order to increase their overall effectiveness. For example, the dynamic fluid seals that must be incorporated between the rotating surfaces of the toolholder shank and tool coupling and the stationary walls of the housing apply a substantial amount of frictional drag which not only increases the power required to effectively turn the cutting tool, but also generates so much heat that the operation of such a toolholder must be stopped every 10 or 15 minutes in order to allow the toolholder to cool off. Even well-designed fluid seals tend to leak substantial amounts of coolant during the operation of the toolholder, not all of which is reclaimable. Also, the centrifugal forces that the diametral bore in the rotating toolholder applies to the coolant flowing into the housing works against the desired flow of coolant out of the tool which in turn requires the use of higher pump pressures than would not otherwise be necessary to obtain the desired coolant flow rate. These higher pressures necessitate tighter fluid seals, which in turn exacerbates the aforementioned frictional drag problem. Finally, the intimate contact made by the coolant flowing through the housing and the bearing surfaces located within the housing necessitates the removal of all metal particles in the reclaimed coolant larger than 5 microns across in order to protect the toolholder shank bearings from damage. Such particulate removal can only be accomplished by the use of a relatively large and expensive filtration unit that disadvantageously applies a substantial back pressure to the coolant flowing in the recirculation path of the system, which again demands higher pump pressures.
Clearly, what is needed is a through-center cooling system for a rotating toolholder that does not require the use of drag-inducing dynamic seals to maintain coolant within the housing. Ideally, such a system would be leak-free so that substantially all of the coolant used would be reclaimable. Finally, such a system would not apply unwanted centrifugal forces to the coolant as it flows to the cutting tool, and would obviate the need for a large and expensive filtration system.